A number of solutions for coating the surface of plastics have been published.
According to U.S. Pat. No. 6,030,662, the coat is formed by an immersion process. In the first step of the process the substrate to be coated is immersed in a solution of a first coating compound having reactive groups and then dried. Then the coated substrate is subjected to an interfacial contact reaction with a second coating composition having at least one low-energy functional group and a reactive functional group that reacts with the functional group in the first coating compound. The interfacial contact reaction between the reactive functional groups in the first and second coating compositions fixes the low-energy functional groups to the substrate surface thereby reducing the surface energy of the substrate surface. In this way a stable binding of the layer is achieved. This method has some disadvantages, e.g. it is laborious, material-intensive and time-consuming. Subsequent stabilisation with a compound having low-energy functional group is not a favourable solution because only that part of the material is utilised which reacts with the inner side of the film layer. The material reacting on the external side or in the interior of the film layer has an adverse effect because it significantly modifies the original properties of the film.
According to U.S. Pat. No. 3,619,246, resin materials having low-surface energy are treated with photosensitizers and ultraviolet radiation. Applying a silane or siloxane, a transitional layer is formed which has a higher surface energy than that of the material to be coated. The transitional layer serves as a basis of the further coating (primarily painting).
According to U.S. Pat. No. 5,039,549, the polyolefin to be coated is soaked in a solution of an aromatic ketone photosensitizer. After removing the solvent, the surface is coated with a second solution comprising (i) a monomer selected from the group consisting of an unsaturated organic acid, an unsaturated organosulfonic acid, a vinyl monomer, (ii) an aromatic ketone photosensitiser and (iii) a low boiling point alcohol or ketone. The coated polyolefin is exposed to irradiation, washed and dried. The layer formed from the monomer makes the polyolefin capable to be glued.
The main disadvantage of the above two processes is the need for an extra device. In addition, carrying out homogeneous ultraviolet radiation is very complicated even with objects of regular shape. The coating can be produced after multistep pre-treatment, only.
According to U.S. Pat. Nos. 5,851,726 and 5,700,559, the surface is activated by ultraviolet irradiation and plasma treatment, respectively. After activation a film-like coating is formed by immersion into a coating material. The coating is bound to the surface by chemical adsorption and by ionic bond, respectively. The disadvantage of these processes is that they allow formation of film-like coatings only, since the binding is formed between the activated surface and the coating material contacting directly the activated surface.
According to U.S. Pat. No. 5,455,108, coating is formed on polymeric fabric in order to reduce the adsorption of proteins by the polymeric fabric. Fabrics are treated with solutions of various surfactants and exposed to corona discharge. The disadvantages of the method are that the coating on the fibres constituting the fabric is inhomogeneous and the process demands an extra device.
According to U.S. Pat. No. 5,583,211, on the surface of polypropylene nucleophile (preferably amino) groups are formed by exposing the polypropylene to plasma energy, then biopolymers (peptides, oligonucleotides, proteins, oligosaccharides) are attached to these groups by covalent bond. Although this process solves the old problem of binding biologically active molecules to indifferent carriers, plasma activation is applicable only in the case of plain surfaces, e.g. sheets, films and fibres.
According to U.S. Pat. No. 5,049,403, a polyamine of high average molecular weight is adsorbed on the surface to be coated and it is stabilized simultaneously with the adsorption or in a separate step by cross-linking with crotonaldehyde. The first layer thus obtained is treated with an anionic polysaccharide (for instance with dextran sulfate) to form a second layer. These steps are repeated by turns. In the last step a polyamine layer without cross-linking is formed. In this way a coating containing free amino groups is formed which may be used to form ionic or covalent bonds. The serious disadvantage of the method is that the layer thus formed is not stable enough, since only a part of the bonds holding the layer together is covalent. In case of forming cross-linking simultaneously with the adsorption, a loose precipitate-like layer is created, while in the two-step process a very thin monomolecular layer is formed. Because of the several repeated steps this process is quite long and difficult.
According to U.S. Pat. No. 5,914,182, the surface of the polymer to be coated is treated with a polymeric surfactant which is then cross-linked thereon to form a first layer. The first layer, through its remaining cross-linking groups, covalently binds to a second layer which has functional groups capable of covalently binding to biologically active substances. The process is quite long, it consists of several steps and is suitable for forming very thin, film-like coatings, only.
It is also known from the literature that coating plastics is a frequent task. The coating can be accomplished by several methods, e.g. by means of spraying, immersion, brushing, adsorption, chemical reaction, plasma energy, nebulisation. Coating neutral plastics is very difficult as the application of both the coating methods based on physical interaction and the coating methods based on chemical interaction are difficult or impossible because of their small surface-energy and the lack of reactive functional groups, respectively. The task is especially difficult if the object to be coated has a geometrical shape other than plain.
JP 48.014471A discloses swelling in a moulded thermoplastic resin with a vinyl monomer, and coating thereon an unsaturated polyester composition and curing. According to this technical solution, a liquid form of a vinyl monomer is absorbed in the termoplastic resin.
U.S. Pat No. 4,876,126A discloses a method of coating wherein a plastic is treated with a solution of a compound having a reactive functional group to form an undercoat and then it is further treated with a water-soluble polymer to covalently bond said reactive functional group with said water-soluble polymer.